Metal Organic Frameworks (MOFs) are crystalline compounds consisting of metal ions or clusters coordinated to often rigid organic molecules to form one-, two-, or three-dimensional structures that can be porous. In some cases, guest molecules can stably enter the pores, thus MOF crystals can be used for the storage of gases such as hydrogen and carbon dioxide. Further, since some guest molecules can enter more easily than others, and the pores can be functionalized to change their chemical properties, this can be used as the basis for separation methodologies. For example, MOFs can be used to make a highly selective and permeable membrane to separate small gas molecules (e.g., CO2 from CH4) or liquid molecules (e.g., hydrocarbons, alcohols, water). Additional applications of MOFs are in catalysis, in drug delivery, and as sensors.
Describing and organizing the complex structures of the large number of available MOFs could be a difficult and confusing task without a logical, unambiguous set of classifications. Indeed, the literature is replete with inconsistent use of terminology and a plethora of abbreviations. Recently, a system of nomenclature has been developed to fill this terminology need. The inorganic sections of a MOF, or secondary building units (SBUs), can be described by topologies common to several structures. Each topology, also called a net, is assigned a symbol, consisting of three lower-case letters in bold. MOF-5, for example, has a pcu net. The database of net structures can be found at the Reticular Chemistry Structure Resource (rcsr.anu.edu.au). Further, the International Union of Pure and Applied Chemistry (IUPAC) is working on suitable terminology that can gain a broad acceptance, while at the same time not deviating too much from the most commonly used nomenclature.
Zeolitic imidazolate frameworks (ZIF) are a subset of metal-organic frameworks. The term “zeolite” was originally coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that upon rapidly heating the mineral “stilbite,” it produced large amounts of steam from water that had previously been adsorbed into the material. Based on this, he called the material zeolite, from the Greek zeo, meaning “boil” and lithos, meaning “stone.”
We now know that zeolites are microporous, aluminosilicate or silicate minerals, and are considered a subtype of MOF crystals. As of November 2010, 194 unique zeolite frameworks were identified, and over 40 naturally occurring zeolite frameworks are known. Some of the more common mineral zeolites are analcime, chabazite, clinoptilolite, heulandite, natrolite, phillipsite, and stilbite.
Zeolites have a porous structure that can accommodate a wide variety of cations, such as Na+, K+, Ca2+, Mg2+ and many others. These positive ions are rather loosely held and can readily be exchanged for others in a contact solution or gas. The regular pore structure and the ability to vary pore size, shape and chemical nature makes zeolites very useful as molecular sieves.
Depending on their structure and composition, zeolites can separate molecules based on adsorption and/or diffusion of certain molecules preferentially inside the pores or exclusion of certain molecules based on their size. The pore size is typically less than 2 nm and comparable to that of small molecules, allowing the use of zeolites to separate lightweight gases such as CO2 and CH4. For example, one liter of ZIF crystals can store about 83 liters of CO2. ZIF crystals are also non-toxic and require little energy to create, making them an attractive possibility for carbon capture and storage. Further, the porous ZIF structures can be heated to high temperatures without decomposing and can be boiled in water or solvents for a week and remain stable, making them suitable for use in hot, energy-producing environments like power plants.
The liquid separation properties of MOF membranes are also attracting increased attention, and are of high interest in a number of emerging applications (e.g., separation of higher hydrocarbons, organics/water separations). Recently, it was reported that polymer/MOF mixed matrix membranes containing ZIF-8 exhibited high selectivity for alcohols over water.
Zeolitic imidazolate framework-90 or “ZIF-90” is one example of an attractive MOF for application in selective membranes. ZIF-90 has a sodalite cage structure with ˜0.35 nm pore windows, through which size exclusion of molecules is possible. Furthermore, the imidazole linker in ZIF-90 contains a carbonyl group, which can have a favorable chemical noncovalent interaction with polar molecules. The structure of ZIF-90 in the [111] direction is shown in FIG. 6A; and the structure of a single unit cell of ZIF-90, showing the one-dimensional channels available for molecular adsorption and diffusion is shown in FIG. 6B.
Zeolitic molecular sieving membranes with very high throughput and high selectivity can be fabricated by hydrothermal processing on flat and tubular ceramic supports.
In making ZIF membranes on aluminum, steel, glass or ceramic supports, a seed layer is deposited and then crystals grown on the seed layer. Various methods have been used to obtain a uniform thin seed layer, include dip-coating, evaporative deposition, rubbing techniques, and waxing techniques. Efforts are also made to control the orientation of the seed layer and the quality of the substrate, and thus the properties of the final membrane.
The experience gained in the preparation of MFI and other zeolitic membranes has shown that, in addition to gross defects in the membrane layer, such as pin holes and cracks, there are many factors critical for the performance of the composite membranes. Some of them are (i) the adhesion of the zeolite layer on the support surface, (ii) the difference of the thermal expansion coefficients of support and zeolite, (iii) the orientation of zeolite crystals, (iv) the thickness of the zeolite layer, (v) the anisotropy of mass transport due to an anisotropic pore geometry, and (vi) the influence of crystal boundaries on the permeation properties.
However, current membranes currently have limited application due to the high cost of the support materials and the difficulties encountered in the scale-up and reliability of hydrothermal growth and subsequent processing steps such as high-temperature calcination to remove the organic structure-directing agents.
Thus, the scalable fabrication of MOF membranes remains a key issue, and a departure from the paradigm of membrane growth on ceramic, aluminum, glass and steel substrates is still needed.
An alternative route to high-performance MOF membranes is to grow them on porous polymer supports, particularly hollow fibers. Macroporous (˜100 nm pore size) polymer hollow fibers can be easily manufactured at low cost, for example, from polyamide-imide polymer (e.g., TORLON®). These fibers can be bundled together to produce low cost hollow-fiber modules with 1000-10,000 m2 of membrane surface area/m3 of module volume. In order to obtain molecular selectivity, it is necessary to find methods to grow continuous MOF membranes on the surfaces of these hollow fibers. These methods must use sufficiently low temperatures and appropriate reagents and solvents so as to not degrade or destroy the polymeric supports.
Little is known regarding the growth of ZIF materials on polymeric surfaces, although our own work suggests that small, uniform seed crystals are desirable for coating on the surface prior to membrane growth (see e.g., U.S. patent application Ser. No. 13/399,645, filed Feb. 17, 2012, and U.S. patent application Ser. No. 13/396,411, filed Feb. 14, 2012).
The techniques used for inorganic supports are not easily transferable to polymeric supports, because of issues of material compatibility, poor adhesion at the polymer-ZIF interface, the highly curved nature of the hollow fiber surface and because the polymer may be unstable at the conditions required for membrane growth. The ZIF often does not bind well to the polymer, creating surface defects that contribute to poor selectivity, and prevent the realization of useful membranes. Incorrect processing conditions may also cause dissolution or collapse of the porous polymer. Further, many of the solvents used are harmful for the polymer.
To solve these problems, some have tried modifying the surface of the zeolite particles or modifying the surface of the polymeric support. Organosilanes, sizing agents, for example, polyetherimide (e.g., Ultem™ (SABIC™), and surface treatments (e.g., Grignard reagent) have been used and have shown some improvement in providing membranes with increased selectivity. The Grignard treatment, for example, involves growing Mg(OH)2 whiskers on the surface, and was originally developed using an aluminosilicate such as zeolite 4A, although it has been extended to MFI by the Nair group at Georgia Tech Research Corp.
Others have tried using dense films made by solution casting. However, such dense films are very slow, and to maximize flux, thinner films are better.
Other alternatives are to create a mixed ZIF/polymer support matrix. See Ge, et al. (2009), for example, which describes making composite hollow fibers by blending zeolite crystals into the polymer feed before the hollow fiber extrusion. See also Bae, et al. (2010). The embedded zeolite crystals catalyst as seeds for the resulting zeolite membrane growth, and they also “anchor” the zeolite membrane to the support to increase the adhesion of the zeolite membrane. However, this method is not cost effective, and not readily scaled up.
Thus, despite intensive research efforts, there remains a need in the art for a scalable, cost-effective method for preparing high-quality ZIF membranes on porous polymeric supports that are technologically scalable. The ideal method would also have general applicability to other MOFs and be useful for a variety of polymers.